Just stumbled by here - my .02. I have a very old Diamondback hybrid - heavy, 700cc, but sturdy. I've ridden it in a couple of "olympic" levels Tris - including the LA before they stopped it. I've done over 50 miles on it, but never a century. It always depends on how comfortable you want to be. I'm a Sr. Cit (barely!) who's had back surgery, so my main concerns are staying upright (had too many road bike caught in street cracks) and keeping comfortable and staying in shape. You will not ride a fast century, but you can more likely ride a safe and comfortable one. Tri bars. Most comfy seat you can find. I like slide-in toe hooks, not clip-ins. Had a guy cut me off on the Balboa bike path once - pulled right in front of me on the gold course exit - and I was able to hit his tire and superman across his hood without much damage (to me or him, lol) and I appreciate that option.

tl:dr - sure you can, but it'll be slow - but could be more comfortable.